The present invention relates to an imaging method using nuclear magnetic resonance (hereinafter referred to as "NMR"), and more particularly to a chemical shift imaging method capable of eliminating an offset phase peculiar to an NMR imaging apparatus.
In an NMR imaging method, it is necessary to discriminate signals generated by nuclear spins in a desired portion of an object, from those in the remaining portion of the object. For this purpose, the object is placed in a predetermined static magnetic field, and a field gradient is applied to the static magnetic field so that the magnetic field intensity varies with position. That is, the resonance frequency or the degree of phase encoding is varied with position, to obtain positional information.
The fundamental principle of the above method for obtaining positional information has been reported by Kumar et al. (Journal of Magnetic Resonance, Vol. 18, 1975, page 69), and further reported by Edelstein et al. ("Physics in Medicine and Biology", Vol. 25, 1980, page 75).
An image indicating the magnetization distribution in an object, for example, a spin density distribution image with respect to the atomic nucleous of hydrogen has been first formed by the above imaging method. Further, a magnetization distribution image for each of frequency components appearing on a frequency spectrum, that is, a spin density distribution image for each of a plurality of chemical shifts has been formed by such an imaging method, in recent years.
A chemical shift is based upon a fact that a magnetic field applied to a nuclear spin is dependent upon a molecular structure around the spin, that is, the resonance frequency of a nuclear spin depends upon the position of the spin in a molecular structure. Thus, the chemical shift is a very important quantity capable of providing information on the molecular structure of an object.
A typical one of chemical shift imaging methods is proposed by Dixon ("Radiology", Vol. 153, 1984, page 189). In the above typical method, a pulse sequence 90.degree.-.tau..sub.a -180.degree.-.tau..sub.b -signal detection is used for obtaining a spin distribution image, and the sum of a first image for .tau..sub.a =.tau..sub.b and a second image for .rho..sub.a .noteq..tau..sub.b and the difference between the first image and the second image are used for obtaining a spin distribution image with respect to a specified chemical shift. In the above pulse sequence, 90.degree. indicates an RF magnetic field for inclining a nuclear spin at 90.degree. with an original state, and 180.degree. an RF magnetic field for inclining a nuclear spin at 180.degree. with an original state. According to this method, a measuring time is only twice longer than a time required for obtaining a single image. Hence, the method is very practical. Further, an improved version of the method has been known, in which the measurement using the above pulse sequence is repeated while varying the time difference .DELTA..tau. between the time intervals .tau..sub.a and .tau..sub.b, and simultaneous equations formed of a plurality of image data which are obtained by the repeated measurement, are solved to obtain a spin distribution image for each of a plurality of chemical shifts. In a case where only two chemical shifts appear, image data is obtained using a pulse sequence in which the time difference .DELTA..tau. is set to a specified value, and the real and imaginary parts of the image data are used for otaining spin distribution images for one and the other of the two chemical shifts. These methods are described in a patent application Ser. No. 846,151 (filed in the United States on Mar. 31, 1986).
In these chemical shift imaging methods, a fact that a difference between two of chemical shifts appears as a phase difference in image data, is utilized to discriminate the chemical shifts from one another. However, the image data contains an offset phase which is peculiar to an NMR imaging apparatus and cannot be removed by the adjustment thereof. Thus, image data obtained by measurement contains phase rotation due to the above offset phase, in addition to phase rotation due to chemical shifts. A reference sample inserted in an RF coil has hitherto been measured to previously detect the offset phase, and image data obtained by measuring an object is corrected using the detected offset phase, to obtain image data which does not contain an offset phase. However, the measurement of the reference sample requires time and labor, and moreover the detected offset phase is led into error by a change in measuring condition.